1. Field of the Invention
In one of its aspects, the present invention relates to the hydrophobizing of particles, particularly inorganic water-insoluble compounds. In another of its aspects, the present invention also relates to the treated particles, pe. The treated particles are useful particularly, but not exclusively in the compounding of polymers, especially rubbers and plastics. In another of its aspects, the present invention also relates to dispersions of the treated particles in a polymer-containing matrix (binder) and to a process for preparing such dispersions.
2. Description of the Prior Art
Raw polymers, either rubbers or plastics rarely have the inherent physical or chemical properties in their pure state that are necessary to make useful articles. The polymers must thus be further compounded by mixing with additional ingredients or "additives". Polymer additives may include one or more of: secondary polymers; extender oils; fillers; antioxidants; coloring pigments, stabilizers, flame retardants, processing aids and other ancillary chemicals. For rubbers, this list may be extended to include curatives (vulcanizing agents) such as sulfur or organic peroxides; cure accelerators such as dibenzothiazyl disulphide (MBTS) and tetramethylthiuram disulfide (TMTD), as well as inorganic cure activators such as zinc oxide, lead monoxide (PbO, litharge), red lead (Pb.sub.3 O.sub.4), and the like. Regardless of whether it is plastic or rubber properties in which improvement is sought, the selected additive materials must be mixed intimately with the polymer at the compounding stage (so as to obtain a homogeneous dispersion) in order for the maximum improvements to be realized. Conventionally, this mixing is usually accomplished on an open mill, in a mixing extruder or in an internal mixer (such as the Henschel, Welex or Banbury types) using one or more steps until the desired degree of dispersion is achieved.
Quite often, a satisfactory dispersion of the additive in the polymer is difficult to attain in a reasonable time, resulting in inhomogeneity which translates into unacceptable physical properties or appearance in the finished compound. To improve the dispersion, an extended mixing time or multi-stage mixing cycle must be employed which lowers productivity in the mixing plant and is thus undesirable.
Within the industry, it is known that mixing of inorganic chemicals such as the inorganic activators used in the rubber industry presents special difficulties in this regard because of the inherent hardness and much higher viscosity of these chemicals relative to the polymer matrix.
One general method of facilitating mixing and dispersion of these inorganic materials into polymer compounds in the factory is to use a very fine particle size inorganic material. However, this inevitably generates dust during both the material handling and mixing process and in many cases these dust particles are toxic or otherwise unacceptable from a worker health standpoint. Dust losses also change the ratio of the chemicals to the base polymer from what was originally intended; this may lead to poor processing or poor finished properties in the compound. In some specific cases (i.e. with talc), very fine particles may act as a lubricant and actually contribute to poor mixing of the bulk, in this case by reducing the shear which is needed for dispersion. In other cases, especially where polar ingredients must be mixed into a non-polar polymer, agglomeration of the particles may occur during mixing, leading to undesirable inhomogeneity and unsatisfactory physical properties.
To mitigate the above problems, it is well known in the art to add the inorganic chemicals to the base polymer in a predispersed form, e.g., as fine particles bound in a low viscosity medium (or binder) such as polymer or oil, or combinations thereof with additional additives. This bound form of inorganic chemicals overcomes the dust problem in the rubber compounding plant and also greatly shortens the dispersing time of the inorganic materials in the polymer compound, particularly if the binder is chemically similar to the base polymer and the viscosity of the pre-dispersion closely matches that of the rest of the compound. From a compounding standpoint, it is desirable to have the minimum amount of binder that will both facilitate dispersion and eliminate dusting during processing.
These types of `concentrates` or `dispersions` thus typically contain from about 50% to 95% by weight of the active inorganic chemical dispersed in a suitable binder (practically, this corresponds to a range of from 100 to 1900 parts by weight of inorganic chemical per 100 parts by weight of binder). Many such materials are commercially available from a number of suppliers to the rubber industries. Non-limiting examples of such commercial polymer-bound materials used in the rubber industry are: RHENOGRAN.RTM. ZnO-85 (85 weight percent zinc oxide dispersed in an EPDM/EVA binder); POLY-DISPERSION.RTM. PLD-90 (90 weight percent lead monoxide dispersed in polyisobutylene); RHENOGRAN.RTM. Pb.sub.3 O.sub.4 -90 (90 weight percent red lead oxide dispersed in EPDM/EVA), all available from Rhein-Chemie Corporation and Rhein Chemie Rheinau GmbH.
Cheaper oil-based binders may also be used; while these address the dust problem, they do not offer as good or as rapid a dispersion as the presence of oil lowers the friction necessary to cause comminution of the inorganic materials during mixing. The presence of oil may also cause other changes in the physical properties (i.e. softening) or appearance (colour) which are undesirable. An example of the latter type of dispersion is Polydex.TM. PPD (ZnO) 75, a 75 weight percent blend of ZnO in a light process oil, available from Polychem.
In the plastics industry, it is often desired to modify the viscosity (i.e., the "melt index"), hardness, color, light-fastness and/or other properties of the base polymer in order to render it processible or suitable for its intended end-use application. Again, these additives (chemicals), in their pure form, may be added directly to the bulk plastic during the processing (compounding) phase; although, it is more customary to use the materials as concentrates in liquid or pellet form in order to obtain better dispersion and better control of the process. Again, these concentrates consist of a dispersion of fine particles of the additive in a suitable carrier or `binder` which may be similar or identical to the base polymer, or it may be another compatible polymer or a combination of polymers and oil. Also, other ingredients (e.g., soaps, compatibilizing agents and dispersing aids) may be included in the base of the binder. This concentrate form is used almost exclusively for introducing inorganic colorants into plastics where the high hardness and high melting point of the additives causes dispersion problems. Many companies currently supply inorganic and organic additive concentrates to the plastics industry; non-limiting examples of the latter materials include: ComPETe.TM., CELPRO.TM., Holoflake.TM., Hanna-FX.TM.(M.A. Hanna Color), BARKOLEN.TM. (SPUR.TM. a.s.), POLYPLUS.TM. (PolyTech South Inc.), CEK CONCENTRATES.TM., COLORPLAST.TM., CONCORDE.TM. (C.E.K. Concentrates) and the like.
Conventionally, these pre-dispersed forms of inorganic additives for use in the rubber and plastics industries have been produced by dry mechanical mixing or the ingredients--i.e., the additive in question is simply mechanically mixed with the binder material. Unfortunately, this approach serves only to transfer the mixing and dust problems from the compounders plant to that of the supplier of the dispersion. Moreover, the relatively high percentage of inorganic material to binder desirable in these dispersions moreover requires long mixing times or the use of special high energy mixing equipment (HIDM) which either lowers productivity or adds to the production costs. What would be most desirable is a dispersion manufacturing process that could be made essentially dust-free and moreover required little mixing energy to disperse the inorganic ancillary material in a polymeric binder.
A facile known method to prepare fine particle size materials from coarser commercially available ones is by wet grinding, using either a ball, colloid or steam jet mill or other equipment as described under "Wet Grinding" in Ullmann's Encyclopedia of Industrial Chemistry Vol. B2 sec. 5-36, the contents of which are hereby incorporated by reference. As the fine particles produced are continuously in a wet state, they have little tendency to become airborne dust. However, the concentration of the fine particles in the wetting medium is of necessity low in order to maintain the fluidity required for satisfactory grinding and thus the particles must be insoluble in the grinding medium. Where media other than water is employed during the size reduction process, additional hazards such as flammability must be taken into consideration. Further, the resulting dispersions typically require concentration (i.e., solvent removal) before they can be further dispersed in a binder. Moreover, it is difficult to dry such fine particles without generating dust elsewhere in the process or without causing agglomeration (particle growth) during the drying step. Where possible, it would be preferable to produce masterbatches, dispersions and concentrates of these particles in suitable binders while the particles are still in a finely divided wet state. It is also preferable for economy and safety perspectives that the grinding medium be water. An additional benefit of using water is that it is generally a non-solvent for most of the organic and inorganic additives which are sold as dispersions.
A number of prior art references teach how to make "masterbatches" of fillers and dispersions of other chemicals in polymers by using fine particles dispersed in an aqueous state. For instance, Burke (U.S. Pat. Nos. 3,689,451, 3,689,452, 3,700,690, 3,716,513 and 3,840,382, the contents of each of which is hereby incorporated by reference) teaches how to use an aqueous dispersion of never-dried alkali silica pigment or a mixture of an aqueous dispersion of never-dried alkali silica pigment silica and carbon black to make a dispersion of these fillers in a matrix of rubbers at levels of &lt;100 phr of filler. The rubbers must be used as solutions in water-immiscible solvents. Typically, large amounts of ancillary chemicals must also be employed to ensure transfer of the silica from the water suspension into the organic phase. In related patents (U.S. Pat. Nos. 3,686,219 and 3,694,398, the contents of each of which are hereby incorporated by reference) Burke teaches how to prepare similar dispersions from finely (aqueous) dispersed particles of silica by using the rubber in the form of a water emulsion--i.e., a latex. Nonetheless, all of the above Burke patents are restricted to the use of never-dried silica or combinations of never-dried silica and carbon black (i.e., conventional rubber fillers). The levels of the inorganic material in the finished dried masterbatch is moreover restricted a low concentration and the binder is restricted to elastomers. To the knowledge of the inventors, no commercial masterbatches made by the Burke methods are currently available.
Contrary to the apparent commercial unavailablity of silica masterbatches, masterbatches of carbon black and rubbers prepared from both water emulsions of polymers (i.e., the latex as resulting from emulsion polymerization) and solutions of polymers in hydrocarbons (i.e., as resulting when the polymer is soluble in the polymerization medium) have been available from several suppliers for a number of years (Copolymer Div. of DSM; Bayer Inc.; Goodyear etc.,). These masterbatches are usually prepared by grinding the carbon black in a wet aqueous state and then intensively mixing the black slurry with a solution-polymer "cement" or emulsion polymer latex, with or without added oil, followed by coagulation and drying. In all commercial products, the levels of black filler are &lt;100 phr.
Where the polymer is available as an aqueous emulsion (i.e., latex), various methods are available for the incorporation of ancillary chemicals to form dispersions; the coprecipitation methods of Leo and Johansson (U.S. Pat. No. 4,110,240) may be used to prepare concentrates containing 80-99.5 wt % of the ancillary chemicals (excluding fillers), either organic or inorganic in the polymeric binder. Kanou et al. (U.S. Pat. No. 4,713,411) detail a different coprecipitation process to produce a pigment composition by using a special water-soluble polymeric binder which is then rendered insoluble by pH changes. However, many polymers, especially plastics, are prepared by a solution polymerization process and are not readily available in a latex form.
Thus, despite previous efforts in the prior art, there remains a need for an efficient manner of producing dispersions or concentrates of inorganic additive materials in a binder wherein the additive is present as the major component (i.e., 50 percent by weight or more) of the dispersion or concentrate and the binder is a polymer which is not readily available in a latex form.